The invention relates to a braking and steering system for a motor vehicle, having a steered front axle and a separate brake circuit for the front axle and the rear axle.
For a better understanding of the terms employed below, some term definitions, on which this application is based, are listed first:
Fail-safe property: [source DIN 19 250] Ability of a technical system, when specific failures occur, to remain in the safe state or to change directly into another safe state; also known as fault safety;
Fault [DIN 19 250]: The non-fulfillment of at least one requirement placed on a necessary feature of a unit under consideration;
Fault tolerance: [NTG 3004; Nachrichtentechnische Gesellschaft [Telecommunications Association] in the VDE [Verband Deutscher Elektrotechniker Federation of German Electrical Engineers] (nowadays ITG, Informationstechnische Gesellschaft [Information Technology Association]), Reliability terms in respect of complex software and hardware, 1982] Ability of a system to perform its specified function even with a limited number of faulty subsystem;
Redundancy: [VDI [Verein Deutscher Ingenieure=Union of German Engineers]/VDE 3542; Safety terms for automation systems, 1991] The presence of more than means necessary per se for carrying out the intended tasks.
Fail-silent: The property of a component or of a system which is in communication with others to transmit no further information when a fault within the component or within the system is detected. Vehicles of the type referred to above are generally known. In addition, German patent doc DE 43 34 260 A1 discloses a fail-safe anti-lock system and power-steering system, in which the control unit of one system monitors the functioning of the other system in each case. When a fault occurs in one of the systems, the other cuts off the same system as soon as is safely possible. In this arrangement, there is always a continuous mechanical or hydraulic connection between the service brake on the individual wheels (usually a disc brake with a hydraulically loaded wheel-brake cylinder) and the brake pedal, which can be randomly actuated by the driver, or between the steering linkage and the steering wheel, which can be randomly actuated by the driver. When a fault occurs in the anti-lock brake system or in the power steering, this mechanical connection ensures that the entire system is fail-safe. When a fault occurs in the anti-brake system, braking can still be carried out, with the function of the anti-lock system being dispensed with. If a fault occurs in the assistance afforded by the power steering, then, although the driver has to exert correspondingly higher steering forces, the vehicle nevertheless remains driveable at least to a restricted extent.
Furthermore, International patent document PCT 94/26558 discloses an arrangement for increasing the reliability of the data transmission between two control units by making the data bus connecting these control units redundant (in this particular case at least twofold).
The object of the present invention is to provide an at least fault-tolerant braking and steering system for a vehicle which requires neither a continuous mechanical connection between the steering wheel and the steered wheels, nor a continuous mechanical and/or hydraulic connection between the brake pedal and the service brake for the wheels.
In particular, another object of the invention is to provide a fault-tolerant braking and steering system such that fault tolerance of the entire system is attained, with as low a redundancy as possible of the individual elements of the system, particularly complicated adjusting systems.
In a generic braking and steering system for vehicle having at least two axles, this object is achieved by the control arrangement according to the invention, in which a braking function is performed on each axle for each wheel, and a steering function is performed on at least one axle. A fault-tolerant, preferably redundant, computing unit determines a desired braking effect, at least for each wheel, and a desired steering effect for each wheel with a steering function, in each case in response to sensor signals. The braking function and the steering function for the wheels are regulated or controlled by adjusting systems based on the determined desired braking effect and desired steering effect. The adjusting system for the braking function contains a service brake, and for the steering function also contains a steering adjuster. A fault-tolerant communication device connects the adjusting systems to the computing unit. The energy supply of the computing unit and of the adjusting systems is designed to be fault tolerant.
The designations "axle" and "wheels of one axle" are employed herein, to connote the paired arrangement of wheels on opposite sides of the vehicle. They do not necessarily mean that a continuous axle (rigid axle) is actually present in the conventional mechanical sense. On the contrary, it may also be a question of individually suspended wheels which are completely independent of one another. In this case, the designation "desired steering effect" embraces the desired steering angle 0.degree. (that is, straight ahead) and the designation "desired braking effect" also embraces the desired braking effect "O" (deceleration-free driving).
It is thus sufficient to design the computing unit, the communication device and the energy supply with fault tolerance, in order to make the entire system (which, furthermore, comprises at least the adjusting systems for the steering function and for the braking function) fault-tolerant.
In vehicles equipped with braking and steering system according to the invention, conventional braking mechanism/hydraulics and the conventional steering mechanism can be dispensed with, without the fault tolerance of the entire system being impaired. That is, at least the braking function and the steering function are controlled or regulated purely electrically, so that driver-assistance systems, such as anti-lock brake systems, traction controls, and other systems which, by appropriate activation of the braking function and the steering function, stabilize the yawing behavior of the vehicle and thus prevent the vehicle from skidding, can be incorporated in the computing unit by the simple implementation of corresponding programs.
Furthermore greater freedom in the design of the vehicle is achieved, so that, for example, the protection of the occupants can be improved, the number of versions of the vehicle reduced (vehicles with right-hand and left-hand drive now differ only in the arrangement of the steering wheel and pedals in the vehicle, but no longer in the arrangement of the adjusting systems for the braking function and the steering function) and the design of the engine space and rear axle simplified.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.